Nuclear Radiation (3): Biological Effects of Radiation
Radiation Doses and Damage
Biological Effects of Nuclear Radiation
Radioactive Fallout: Strontium 90
In this third part on nuclear radiation I want to mention some of the biological effects. Although many agree that no level of radiation is safe, it really comes down to a matter of chance. Since negative effects of radiation increase with increased exposure, we should do what we can to keep radiation around us to a minimum.
No Nuclear reactor could explode like an atomic weapon, but there are important lessons from the period of atmospheric testing. After atomic weapons were developed in WW II, the USA and the former Soviet Union started to explode nuclear weapons in the atmosphere.
Not surprisingly the levels of radiation started to climb in the world until 1963 when the United Nations agreed to stop testing in the atmosphere. Why? When Uranium 235 undergoes fission, it produces a number of smaller elements.
What are those smaller fragments? Certainly there are neutrons, beta particles, gamma rays and a large production of energy. In addition there are elements and these are radioactive isotopes and fission products. I got this list from Wikipedia
Notice there are both long and short lived species. Although long lived products stay around a long time, they emit radiation slowly and cause fewer problems than the medium lived products. They decay more quickly and produce more radiation in a shorter time. All these elements are unnatural in our environment, and there are too many to discuss, so I will focus on one, Strontium 90.
It has a half life of 28.9 years (for comparison, Uranium-235 has a half life of 704 million years, and for U -238 it is 4.5 billion years). That is if you start with 1 gram of Uranium 235 and one gram of Sr-90, in 28.9 years we have half a gram of strontium 90, but it takes 704 million years to get half a gram of U-235.
So strontium 90 decays relatively quickly; has a relatively high yield, which means a it is a major fission product; it is a beta emitter with a large decay energy. So in the period of nuclear testing, the levels of Strontium 90 started to climb around the world.
Notice Strontium lies one period below calcium on the periodic table. It therefore has properties similar to calcium, which is essential for life and a major component of bones. That is, strontium 90 can be taken up by the body and replaces calcium.
Starting in the 1950’s, strontium 90 levels increased. The map (in the video 3a) shows the distribution of Sr 90 fallout in the USA from the Nevada Test Site. You can see that the wind blows from West to East and is distributed far and wide. It got into dairy products and into our bones, especially those of growing children. When it decays, the large energy emitted causes damage and can lead to bone cancer. Since the banning of atmospheric testing, and with a half life of 30 years the Sr-90 concentration in our environment is dropping, being about 20% of what it was before the test ban in 1963.
Strontium 90, and many other radioactive elements are also by-products of nuclear reactions. That is why it is so important to keep them contained and why a reactor meltdown presents serious health hazards. Recall that The Three Mile Island partial meltdown released no significant amounts of radiation, but the Chernobyl meltdown certainly did lead to wide contamination and contributed to the deaths and birth defects of many thousands of people.
The recent Japanese earthquake caused major damage at the Fukushima Nuclear Plant, and released radiation into the environment. At the time of writing this, a meltdown is suspected but has not yet been declared as one by officials.
The release of radiation is the main reason why the world is so concerned about the crisis in Japan
Radiation Doses and Damage
As radioactive particles travels through the body, they interact with molecules in the tissues, breaking bonds and ionizing atoms because of the enormous amount of energy carried by radioactive particles.
A somatic cell is any one of the structural or functional cells of an organism. All cells that are not reproductive in function are somatic cells (e.g., sperm is not somatic).
When a stream of radioactive particles enters living tissue, it may damage molecules such as enzymes and structural molecules that are vital to the workings of the cells. The skin cells can die and this is what causes radiation burns. When so many cells die the workings of internal organs are hampered and this is the cause of radiation sickness . Somatic damage may be immediately lethal, kill over a period of time, or be slight enough to be repaired by the body, depending on the extent of the damage. Effects of mild radiation may not appear for years. For example leukemia rates increase after periods of about 20 years.
Genetic damage occurs to our generative cells that can lead to cancers or genetic defects passed onto our offspring as birth defects.
Exposure to radiation is measured in terms of the amount of energy carried by the radioactive particles which can be transferred to living tissue.
A rad is a radiation absorbed dose and is the standard unit used to describe the amount of energy transferred from a stream of radioactive particles to living tissue. It is equal to 0.01 J of energy absorbed by a kg of tissue. Another unit is the Gray (Gy) and a dose of 1 Gy is 1 J/kg.
Therefore, 100 rad = 1 Gy.
Different forms of radiation, however, affect tissue in different ways. The basic unit of measurement for the dose of radiation received by living human tissue is the rem (rad equivalent for man (humans)). This simply takes into account the actual effect of radiation in addition to the dose received.
A rem is given by the simple equation:
Effective dose (in rem) = dose (in rad) x quality factor (QF)
That is the rem unit takes into account both the energy of the radiation and its relative effect on human tissue. For example alpha particles have a quality factor of 20 and are much more dangerous than X-rays with a quality factor of 1.
A sievert (Sv) is the SI unit for a rem, and is given by the dose (in Gy J/kg) multiplied by the quality factor (QF). The Gray is also an SI unit, but without the quality factor.
Biological Effects of Nuclear Radiation
While the same amount of energy may be deposited by different kinds of particles, the damage done to living tissue can vary. For example, the damage done by an alpha particle vs. a beta particle is quite different. However both rems and Sieverts are large units. A lethal dose corresponds to between 4 to 5 Sieverts. This is about 15,000 times greater than our normal exposure to background radiation in our environment.
Because of its relatively high charge and mass, the alpha particle creates a wide swath of damaged. Alpha particles cause more damage than other particle, and have the largest quality factor of 20.
Lighter particles, such as beta particles and gamma rays, cause relatively little damage to cells when compared to alpha particles.
Because they do not interact as strongly with tissue, their energy does not dissipate quickly, and they can therefore penetrate into much deeper layers of tissue, and pass right through.
The extent of the damage done to the body depends on the type and location of the tissues affected. This is especially important in the case of highly damaging alpha-radiation: if it impacts the skin, it will penetrate only the first few layers of cells, and causes radiation burning.
If an alpha-emitter enters the body it can do damage to vital internal organs.
Gamma rays are dangerous for the following reason. While a gamma ray does not interact strongly enough to cause damage similar to an alpha- or even a beta-particle, it is capable of penetrating much more deeply. A gamma source is therefore able to damage vital internal organs, and cause genetic damage.
We are exposed to small amounts of radiation every day from natural sources in our environment. However, these levels are normally so small as to have no significant effect.
The average yearly dose of radiation for a human being in the First World is 360 mrem or 0.360 mSv. If a lethal dose is say 3.6 Sv, then our background exposure is a 10,000 less than a lethal dose. This chart shows the breakdown of exposure for the average person.
Our daily exposure to radiation is quite low. The largest exposure is from Radon gas and this occurs naturally. In fact naturally occurring radiation accounts for about 70% of our total normal exposure. Since we have evolved in the presence of small amounts of radiation, the effects are small. However radiation of the sort we encounter in Nuclear reactor meltdowns and nuclear weapons can seriously increase those levels.
Well I hope this helped to give some useful background to some of the consequences are of a Nuclear reactor meltdown.
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